With a control device such as temperature controllers, programmable logic controller, and so forth, a time-proportional output is used as one of outputting methods. With the time-proportional output, the magnitude of an output is expressed in terms of an ON-time ratio within a cycle time.
FIG. 5 is a block diagram showing a configuration of a conventional control device, and FIG. 6 is a diagram for describing an operation of a conventional time-proportional driver.
In FIG. 5, a control device 10 controls a control object 20 according to a ratio of ON-time to OFF-time of time-proportional output. A PID computation unit 11 executes PID computation of deviation between a set value SV and a measured value PV from the control object 20, thereby delivering a PID computation output as a control output value MV. A time-proportional driver 12 delivers a time-proportional output corresponding to the control output value MV to the control object 20 upon receiving the control output value MV.
In FIG. 6, 10 ms (in the case of 50 Hz) or 8.666 ms (in the case of 60 Hz), corresponding to one half of a commercial power supply cycle, is often selected as resolution of ON-time, and a cycle time is decided in consideration of a minuscule value expressed in terms of the control output value MV.
For example, in the case where a power supply of 50 Hz is in use, and the control output value MV is expressed by the step of 0.1% in a range of 0 to 100%, a cycle time of at least 10 s (1000 times 10 ms) is required in order to express the time-proportional output in terms of the same minuscule step. When the cycle time is set to 10 s, according to the time-proportional output with the control output value at 40%, the control device 10 is ON for 4 s (Ton), and is OFF for 6 s (Toff).
For example, in JP 2000-47703 A, there has been disclosed a time-proportional type control device capable of accurately and speedily providing a control object with results of output control for controlling the control object.